Replace `CPL_DLL` definition with `CTB_DLL`

Using `CPL_DLL` requires changing GDAL's `cpl_port.h` file which is a hack too
far on many systems so the change is encapsulated in `CTB_DLL`.  This also
re-enables CMake directives that are appropriate for toolchains other than
Visual Studio.
Merge branch 'homme-msvs-2013-compile'